The instant invention relates generally to adjustable torque limiting assemblies and more specifically to adjustable torque transmitting assemblies which are capable of providing an indication or signal that the preselected torque threshold has been reached.
Mechanical assemblies designed to determine torque throughput and utilize such determination to limit such torque by slip or by terminating the flow of energy to an associated prime mover are common power train components. Such assemblies, for example, are an integral portion of power tools which are utilized to tighten fasteners, secure threaded components and generally tighten coupling mechanisms to a desirable level of torque. For purposes of illustration, a specific example is known as a nut runner; a device having an electric or, more typically pneumatic motor which drives a through gear reduction device to tighten threaded fasteners such as bolts, nuts and the like. In such applications, it is desirable to quickly, repeatedly and accurately tighten the fastener such that the associated components are secured together with a necessary, predetermined force. Numerous approaches have been taken, particularly with pneumatic motors to achieve this goal, motor back pressure sensors, stall torque controllers and strain gauges coupled to associated electronics and controls, to name but three. Mechanical devices such as back pressure sensors typically suffer from an inability to be readily adjusted whereas such electronic devices may be delicate and complex.
A second drawback of all systems wherein operation of the prime mover continues until the sensed torque achieves a predetermined threshold at which time energy flow to the prime mover is terminated, is that in such devices, almost without exception, the drive components such as the motor and gear train are directly coupled to the driven fastener. In this situation, in spite of the fact that the monitoring device has sensed attainment of the torque limit and terminated energy flow to the motor, the momentum of the drive train will be transmitted to the fastener, generally over-torquing it. Initially, it would seem that this difficulty could be corrected to simply lowering the required torque threshold by an amount commensurate with the momentum energy. However, the rate at which the motor decelerates as it approaches the torque limit due to the hardness or softness of the joint will effect the momentum energy applied to the fastener and render such a simple compensation scheme generally ineffective.
It thus becomes apparent that an improved torque limiting assembly will both permit ready adjustment of the torque transmission limit as well as providing a means whereby all or a major portion of the drive train energy can be prevented from reaching the driven device. The following is a description of such an adjustable torque limiting assembly.